ath79: add support for KuWfi CPE830(D) / YunCore CPE830(D)

ath79: add support for KuWfi CPE830(D) / YunCore CPE830(D)

Short specification:
* 650/600/216 MHz (CPU/DDR/AHB)
* 2x 10/100 Mbps Ethernet, passive PoE support
* 64 MB of RAM (DDR2)
* 16 MB of FLASH
* 2T2R 2.4 GHz with external PA, up to 30 dBm (1000mW)
* 2x internal 14 dBi antennas
* 8x LED, 1x button
* No UART on PCB on some versions
* Display panel with 2x buttons (F/N) not supported (and not relevant in OpenWrt)-

Flash instructions
* Connect PC with 192.168.0.141 to WAN port
* Install a TFTP server on your PC ('atftp' is doing the job for instance)
* Copy your firmware in the TFTP folder as upgrade.bin
* Power up device pushing the 'reset' button
* The device shall upload upgrade.bin, install it and reboot
* Device shall be booting on 192.168.1.1 as default

Signed-off-by: Joan Moreau <jom@grosjo.net>
Link: https://github.com/openwrt/openwrt/pull/17279
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>